The invention relates to a synchronous operation system for retaining rotating positions of two rotating elements driven by respective induction motors in a predetermined positioning relation making use of a position adjuster, that is, for performing a synchronous control.
In machine tools and the like, rotating positions of two rotating elements may be held in a predetermined positioning relation, so called, a synchronous control may be carried out.
FIG. 3 shows an example of the system, wherein synchronous controls of a spindle on a master side and a spindle on a slave side, as the rotating elements, are carried out by using induction motors and inverters for performing a primary frequency control of the induction motor.
In the same drawing, a synchronous speed command value is applied to an inverters 2 on the master side as a speed command value and to an inverters 3 on the slave side as a speed standard value through a soft start-stop circuit 1 for performing smooth acceleration and deceleration at the time of starting or stopping the system. Induction motors IM.sub.1 and IM.sub.2 are controlled by the speed command value and the speed standard value, whereby a spindle MSP on the master side and a spindle SSP on the slave side are driven. Also, the position detections of the respective spindles MSP and SSP are carried out by a position detector PE.sub.1 on the master side and a position detector PE.sub.2 on the slave side, which are formed of pulse encoders and provided at respective shafts. Output signals of the master side position detector PE.sub.1 and the slave side position detector PE.sub.2 are respectively applied to a master side speed detecting circuit 4 and a slave side speed detecting circuit 5, and the output of the master side speed detecting circuit 4 is applied, as a speed setting value and the output of the slave side speed detecting circuit 5 is applied, as a speed detecting value, to a speed adjusting circuit 6 for calculating the speed variations of the respective spindles MSP and SSP.
The speed standard value of the slave side inverter 3 is added to or deducted from the output of the speed adjusting circuit 6 at an arithmometer 9, and, based on the obtained value, the rotation speed of the slave side spindle SSP is caused to coincide with the rotation speed of the master side spindle MSP through the inverter 3 and the induction motor IM.sub.2.
On the other hand, a position adjuster 7 is applied with output signals of the master side position detector PE.sub.1 and the slave side position detector PE.sub.2, and the position variation of the respective spindles MSP and SSP is calculated. The output of the position adjuster 7 is added to or deducted from the speed standard value of the slave side inverter 3 at an arithmometer 8. By this, retaining the respective spindles MSP and SSP in a predetermined positional relation, what is called, the synchronous control, is carried out.
In the synchronous operation system as described above, in case the synchronous operation is carried out in an unstable region inherent in the induction motor driven by the inverter, or in a specific speed region of resonance point of a machine system and the like, for example, when irregularities of the rotation in the master side induction motor IM.sub.1 occur in the specific speed region, the position adjuster 7 can not follow the irregularities, and the output of the position adjuster 7 keeps approximately constant, so that there has been such a problem that the position variation between the respective spindles MSP and SSP becomes excessively large.
Furthermore, in case the inventer conducts the primary frequency control of the induction motor, when the induction motor is operated in a high speed region (constant output region) exceeding the basic rotation speed (base speed), and the output of the position adjuster is constant with respect to the position variation, the inverter simply conducts the frequency control instead of the speed control, so that as shown in FIG. 4, the induction motor has a characteristic such that the slippage thereof expands from S to S' as the speed increases. In this case, the position variation can not be compensated sufficiently in the above stated synchronous operation system.
Incidentally, in a vector control inverter having a speed control system, a speed command value is compensated in the interior, so that the above stated problem at the high speed region does not occur. That is, the above problem is considered to be specific for inverters for conducting the primary frequency control of the induction motor where the frequency command value is compensated to carry out the synchronous operation.
The present invention has been made to solve the above stated problems, and the object of the present invention is to provide a synchronous operation system wherein the position variation of two rotating elements including a case where the synchronous control is carried out by an inverter controlling the primary frequency of the induction motor, is positively compensated and maintained in a predetermined positional relation, what is called, a synchronous control can be made.